1. Field of the Invention
The present invention relates to a control device for controllably switching the drive mode of a front-and-rear wheel drive vehicle to a drive mode commanded to be switched to by electrically controlling a drive power transmission device installed on the front-and-rear wheel drive vehicle.
2. Discussion of the Related Art
Heretofore, as described in Japanese unexamined, published patent application No. 2001-82571, there have been known a front-and-rear wheel drive vehicle of the type having a driving power transmission device on a driving power transmission path for transmitting the driving power from a driving power source to driven wheels such as, for example, rear wheels. The driving power transmission device installed on the front-and-rear wheel drive vehicle is provided in the driving power transmission path extending from the primary driven wheels to the secondary driven wheels and operates to transmit the driving power to the secondary driven wheels. The driving power transmission device may otherwise be installed inside of a center differential which distributes the driving power to both pairs of driven wheels and operates as a limited slip differential.
The aforementioned front-and-rear wheel drive vehicle is provided with a control device for electrically controlling the driving power transmission device. The control device is capable of selectively switching the drive mode of the vehicle to either of various driving modes by electrically controlling the driving power transmission device. With such a control device, the drive mode of the vehicle can be controllably switched to, for example, two-wheel drive mode, automatic four-wheel drive mode, four-wheel drive lock mode, or the like.
However, the aforementioned control device in the prior art takes the construction that when a driver or the like applies a switching command signal to the control device to switch the drive mode from the present one to a desired one, the switching to the commanded drive mode is effected instantaneously regardless of the state of the vehicle at that moment. Thus, it may be the case that in dependence on the state of the vehicle at the time of such a switching command given, an excessive driving force (i.e., torque) may be transmitted instantaneously to the driven wheels. This undesirably results in the occurrence of a torque exceeding an allowable level, the generation of an abnormal sound or noise, the abrupt change in the vehicle motion or the like.
For example, while the vehicle traveling at a low speed in the two-wheel drive mode is suffering a large slip, the switching to the four-wheel drive mode may cause the torque to be transmitted to the secondary driven wheels abruptly. This may make a cause to the generation of a torque exceeding the allowable level or an abnormal sound. Furthermore, when the torque is changed abruptly by the instantaneous switching of the drive mode during the vehicle traveling, the motion of the vehicle suddenly changes, especially where the vehicle is making a turn.